Calibration of vector modulators using a scalar detector

Abstract

Calibration of a vector modulator is done using a scalar detector to measure the amplitude of the RF output signal, phase shifters to adjust the relative phases of the I and Q components of the RF carrier, and variable attenuators in the I and Q modulation signal input lines to adjust the relative amplitude of the modulation signals. DC signal sources provide reference signals for the I and Q modulation inputs, carrier leak compensation signals, and calibration signals for balancing the amplitude of the I and Q modulation signals. An iterative four step calibration process is followed until no change in the results is observed. The quadrature phase error is minimized by adjusting the phase shifters. The carrier leakage is minimized by adjusting the carrier leak compensation sources to minimize RF output with the modulation inputs grounded. The amplitudes of the I and Q modulation signals are balanced by adjusting the attenuators until the output amplitudes are equal. Finally, the quadrature calibration signal sources are adjusted until the output amplitudes they produce are balanced.

We claim: 
     
       1. A method of calibrating a vector modulator having an input RF source connected to an input power divider providing two outputs, each connected to an adjustable phase shifter, the output of each adjustable phase shifter is independently connected to a mixer, one of the mixers is connected to an in-phase (I) signal path and the other mixer is connected to a quadrature (Q) signal path, the outputs from the mixers are connected to a power combiner, the power combiner produces an output RF signal having an amplitude, the method of calibrating requiring only the measuring of the amplitude of the output RF signal with a scalar detector connected to the RF output signal, the method comprising the following steps: (a) minimizing quadrature phase error;   (b) minimizing carrier leakage;   (c) balancing amplitudes of I and Q modulated signals;   (d) balancing the amplitudes of quadrature calibration signals comprising: a positive quadrature DC reference signal (Qc+);   a negative quadrature DC reference signal (Qc-);   a positive in-phase DC reference signal (Ic+);   a negative in-phase DC reference signal (Ic-); and     (e) repeating steps (a) through (d) until no further changes are observed.   
     
     
       2. The calibration method of claim 1 wherein: step (a) is performed by sequentially connecting combinations of positive DC reference signals Qc+, IC+, and negative DC reference signals Qc-, Ic-, to the I and Q mixers in place of the modulation signals, measuring the RF output amplitude for each of the four combinations of reference signals, and adjusting phase shifters located between the power divider and the mixers so that the sum of the RF output amplitudes produced by the (Qc+,Ic+) and (Qc-,Ic-) combinations is equal to the sum of the RF output amplitudes produced by the (Qc+,Ic-) and (Qc-,Ic+) combinations;   step (b) is performed by connecting carrier leak compensation sources to the I and Q mixers with the modulation inputs grounded, measuring the RF output amplitude and adjusting the carrier leak compensation sources to minimize RF output amplitude;   step (c) is performed by alternately grounding the I and Q modulation inputs while measuring the RF output amplitude, and adjusting attenuators connected between the modulation signal inputs and the mixers until the RF output amplitudes produced are equal; and   step (d) is performed by alternately applying the Qc+ and Qc- signals to the Q mixer with the I modulation signal connected to the I mixer, measuring the RF output amplitude and adjusting the amplitude of one of the Qc+ or Qc- signals until the output amplitudes they produce are equal, then alternately applying the Ic+ and Ic- signals to the I mixer with the quadrature modulation signal connected to the Q mixer, measuring the RF output amplitude and adjusting the amplitudes of the Ic+ or Ic- signals until the output amplitudes they produce are equal to the output amplitude produced by the Qc+ and Qc- signals.   
     
     
       3. The calibration method of claim 2 wherein the RF output amplitude is minimized in step (b) by: measuring the RF output while varying the amplitude of the carrier leak compensation signal;   selecting a first carrier leak compensation interpolation amplitude near one side of the RF output minimum, in a region where the RF response has measurable slope, producing a first RF output amplitude;   selecting a second carrier leak compensation interpolation amplitude near the other side of the RF output minimum, in a region where the RF response has measurable slope, producing an RF output amplitude equal to the first RF output amplitude; and   setting the amplitude of the carrier leak compensation signal at the average of the first and second interpolation amplitudes.